High-capacity digital versatile discs (DVDs) capable of recording at a high density and DVD devices for playing such DVDs have been commercialized and have attracted attention as products of growing demand. Due to the high density recording of the DVDs, an AlGaInP (aluminum gallium indium phosphide)-based semiconductor laser device having an emission wavelength of 650 nm is used as a laser light source for recording and playing the DVDs. Accordingly, an optical pickup of a conventional DVD device can neither record nor play recordable compact discs (CDRs) which are recorded and played by using an AlGaAs (aluminum gallium arsenide)-based semiconductor laser having an emission wavelength of 780 nm.
An optical pickup having lasers of two wavelengths mounted therein has therefore been employed. In this optical pickup, a 650 nm-band AlGaInP-based semiconductor laser (a red laser) and a 780 nm-band AlGaAs-based semiconductor laser (an infrared laser) are mounted as a laser chip in separate packages. A device capable of recording and playing both DVDs and CDRs has thus been implemented.
Such an optical pickup, however, has a large size because two separate packages of the AlGaInP-based semiconductor laser and the AlGaAs-based semiconductor laser are mounted. Accordingly, the size of a DVD device using such an optical pickup is increased.
In view of this problem, an integrated semiconductor light emitting device integrating a plurality of kinds of semiconductor light emitting elements is known in the art. In this integrated semiconductor light emitting device, the plurality of kinds of semiconductor light emitting elements have different emission wavelengths from each other, and the light emitting element structure of each semiconductor light emitting element is formed by semiconductor layers grown on the same substrate. An example of such an integrated semiconductor light emitting device is described in Japanese Patent Laid-Open Publication No. H11-186651 (hereinafter, referred to as Document 1).
FIG. 9 shows an example of the integrated semiconductor light emitting device described in Document 1. As shown in FIG. 9, in a conventional integrated semiconductor laser device 100, a 700 nm-band (e.g., 780 nm) AlGaAs-based semiconductor laser LD1 and a 600 nm-band (e.g., 650 nm) AlGaInP-based semiconductor laser LD2 are integrated in a separated state on the same n-type GaAs (gallium arsenide) substrate 101.
For example, a substrate having a (100) orientation or a substrate having a surface tilted at, for example, 5° to 15° from a (100) face as a main surface is used as the n-type GaAs substrate 101.
In the AlGaAs-based semiconductor laser LD1, an n-type GaAs buffer layer 111, an n-type AlGaAs cladding layer 112, an active layer 113 having a single quantum well (SQW) structure or a multiple quantum well (MQW) structure, a p-type AlGaAs cladding layer 114, and a p-type GaAs cap layer 115 are sequentially formed in this order on the n-type GaAs substrate 101.
An upper part of the p-type AlGaAs cladding layer 114 and the p-type GaAs cap layer 115 form a stripe shape extending in one direction. An n-type GaAs current confinement layer 116 is formed on both sides of the stripe portion, whereby a current confinement structure is formed. A p-side electrode 117 is provided on the stripe-shaped p-type GaAs cap layer 115 and the n-type GaAs current confinement layer 116 and has ohmic contact with the p-type GaAs cap layer 115. For example, a Ti/Pt/Au (titanium/platinum/gold) electrode is used as the p-side electrode 117.
In the AlGaInP-based semiconductor laser LD2, an n-type GaAs buffer layer 121, an n-type AlGaInP cladding layer 122, an active layer 123 having an SQW structure or an MQW structure, a p-type AlGaInP cladding layer 124, a p-type GaInP (gallium indium phosphide) intermediate layer 125, and a p-type GaAs cap layer 126 are sequentially formed in this order on the n-type GaAs substrate 101.
An upper part of the p-type AlGaInP cladding layer 124, the p-type GaInP intermediate layer 125, and the p-type GaAs cap layer 126 form a stripe shape extending in one direction. An n-type GaAs current confinement layer 127 is formed on both sides of the stripe portion, whereby a current confinement structure is formed. A p-side electrode 128 is provided on the stripe-shaped p-type GaAs cap layer 126 and the n-type GaAs current confinement layer 127 and has ohmic contact with the p-type GaAs cap layer 126. For example, a Ti/Pt/Au electrode is used as the p-side electrode 128.
An n-side electrode 129 is provided on the back surface of the n-type GaAs substrate 101 and has ohmic contact with the n-type GaAs substrate 101. For example, an AuGe/Ni (gold-germanium/nickel) electrode or an In (indium) electrode is used as the n-side electrode 129.
The p-side electrode 117 of the AlGaAs-based semiconductor laser LD1 and the p-side electrode 128 of the AlGaInP-based semiconductor laser LD2 are respectively soldered on a heat sink H1 and a heat sink H2 by AuSn (gold-tin) or the like. The heat sink H1 and the heat sink H2 are provided on a package base so as to be electrically isolated from each other.
In the conventional integrated semiconductor laser device 100 described above, the AlGaAs-based semiconductor laser LD1 can be driven by applying a current between the p-side electrode 117 and the n-side electrode 129. The AlGaInP-based semiconductor laser LD2 can be driven by applying a current between the p-side electrode 128 and the n-side electrode 129. Laser light of a 700 nm band (e.g., 780 nm) can be obtained by driving the AlGaAs-based semiconductor laser LD1, and laser light of a 600 nm band (e.g., 650 nm) can be obtained by driving the AlGaInP-based semiconductor laser LD2. Whether the AlGaAs-based semiconductor laser LD1 or the AlGaInP-based semiconductor laser LD2 is driven is selected by, for example, switching an external switch.
As described above, the conventional integrated semiconductor laser device 100 has the 700 nm-band AlGaAs-based semiconductor laser LD1 and the 600 nm-band AlGaInP-based semiconductor laser LD2 on the same substrate. Accordingly, laser light for DVDs and laser light for CDs can be independently obtained. Playing and recording of both DVDs and CDs is therefore enabled by mounting the integrated semiconductor laser device 100 as a laser light source on an optical pickup of a DVD device.
The respective laser structures of the AlGaAs-based semiconductor laser LD1 and the AlGaInP-based semiconductor laser LD2 are formed by semiconductor layers grown over the same n-type GaAs substrate 101. Therefore, only one package is required for this integrated semiconductor laser device. This enables reduction in size of an optical pickup and therefore reduction in size of a DVD device.
This conventional integrated semiconductor laser device 100 has an isolation groove 140 so that the AlGaInP-based semiconductor laser LD2 as a red semiconductor laser and the AlGaAs-based semiconductor laser LD1 as an infrared semiconductor laser have the same chip width. The isolation groove 140 is formed by etching or the like and electrically isolates a red semiconductor laser portion and an infrared semiconductor laser portion which are formed on the same substrate by crystal growth.
In general, a semiconductor laser has characteristics in that its optical output reduces with increase in temperature. It is therefore necessary to sufficiently release the heat generated by the semiconductor laser itself during driving of the semiconductor laser. In order to implement such sufficient heat release, a semiconductor laser is mounted junction-down on a high thermal-conductivity heat sink. It is obvious that the larger the contact area of the semiconductor laser with the heat sink is, the more the heat is released.
In a two-wavelength semiconductor laser device, however, two semiconductor lasers are electrically connected to each other if merely arranged side by side in contact with each other. In order to avoid such electric connection between the semiconductor lasers, it is necessary to form an isolation groove between the two semiconductor lasers. However, a two-wavelength semiconductor laser device having an isolation groove has the following problem. It is herein assumed that a two-wavelength semiconductor laser device having an isolation groove is fabricated with the same dimensions as those of a two-wavelength semiconductor laser device having two semiconductor lasers arranged side by side with no isolation groove. In this case, the heat release area of the two-wavelength semiconductor laser device having an isolation groove is smaller than that of the two-wavelength semiconductor laser device having no isolation groove by the area of the isolation groove. This is because the isolation groove cannot contribute to heat release. As a result, the heat release efficiency is degraded.
If the area of each semiconductor laser is increased to improve heat release, characteristics of the two-wavelength semiconductor laser, that is, reduction in size, will be lost.
In the case where an isolation groove is provided to reduce the heat release area, the resultant degradation in heat release efficiency occurs significantly in a red laser. This is because the step (ΔEc) of the conduction band energy at the interface between an active layer and a p-type cladding layer is smaller in a red semiconductor laser than in an infrared semiconductor laser. In other words, since ΔEc is small, the red semiconductor laser is more susceptible to carrier overflow, that is, a phenomenon in which carriers injected into an active layer are thermally excited and overflow into a p-type cladding layer. As a result, in the red semiconductor laser, saturation of the highest optical output due to thermal saturation is more likely to occur during high temperature operation, as compared to the infrared semiconductor laser.
In order to record DVDs at a 16-fold speed or higher, high output of at least 350 mW is required at a high temperature of 85° C. Such optical output saturation due to thermal saturation therefore causes critical problems.
In a semiconductor laser device disclosed in Japanese Patent Laid-Open Publication No. 2002-190649 (hereinafter, referred to as Document 2), an isolation groove is positioned so that each semiconductor laser has a different electric connection area with another device.
An example of such a two-wavelength semiconductor laser device is shown in FIG. 10. A semiconductor laser device 301 of FIG. 10 includes a 650 nm-band red semiconductor laser (first semiconductor laser) 303 and a 780 nm-band infrared semiconductor laser (second semiconductor laser) 304 on the same n-type GaAs substrate 302. The red semiconductor laser 303 is made of an AlGaInP-based material and the infrared semiconductor laser 304 is made of a GaAs-based material. An isolation groove 305 is provided between the red semiconductor laser 303 and the infrared semiconductor laser 304.
Note that, in the red semiconductor laser 303, an n-type AlGaInP cladding layer 306, an active layer 307, and a p-type AlGaInP cladding layer 308 are sequentially formed on the n-type GaAs substrate 302. The active layer 307 is made of AlGaInP and GaInP and has a multiple quantum well structure. An n-type AlInP (aluminum indium phosphide) current block layer 309 is formed in the p-type AlGaInP cladding layer 308 so as to form a stripe-shaped current path. A structure for confining a current injected into the active layer 307 is thus formed.
A p-type electrode 315 is formed on the cladding layer 308. The p-type electrode 315 is connected to a heat sink (heat dissipating member). The contact area of the p-type electrode 315 with the heat sink is S1 (=cavity length L1×width W1).
In the infrared semiconductor laser 304, an n-type AlGaAs cladding layer 310, an active layer 311, and a p-type AlGaAs cladding layer 312 are sequentially formed on the substrate 302. The active layer 307 is made of AlGaAs and GaAs and has a multiple quantum well structure. An n-type AlGaAs current block layer 313 is formed in the p-type AlGaAs cladding layer 312 so as to form a stripe-shaped current path. A structure for confining a current injected into the active layer 311 is thus formed.
A p-type electrode 316 is formed on the cladding layer 312. The p-type electrode 316 is connected to a heat sink. The contact area of the p-type electrode 316 with the heat sink is S2 (=cavity length L2×width W2). Note that L1=L2.
In the semiconductor laser device 301 described above, the red semiconductor laser 303, which is smaller in thermal conductivity of the p-type cladding layer and ΔEc than the infrared semiconductor laser 304, has a wider chip width W1 than a chip width W2 of the infrared semiconductor laser 304. Since the respective cavity lengths L1 and L2 of the red semiconductor laser 303 and the infrared semiconductor laser 304 are equal to each other, the respective contact areas with the heat sink, that is, the respective areas contributing to heat release, have the relation S1>S2. Accordingly, excellent temperature characteristics can be implemented both in the red semiconductor laser 303 and the infrared semiconductor laser 304.